perf build: Allow passing extra Clang flags via EXTRA_BPF_FLAGS

Add support for EXTRA_BPF_FLAGS in the eBPF skeleton build, allowing
users to pass additional clang options such as --sysroot or custom
include paths when cross-compiling perf.

This is primarily intended for cross-build scenarios where the default
host include paths do not match the target kernel version.

Example usage:

    make perf ARCH="arm64" EXTRA_BPF_FLAGS="--sysroot=..."

Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: hupu <hupu.gm@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <nick.desaulniers+lkml@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -35,6 +35,9 @@ include ../scripts/utilities.mak
 #
 # Define EXTRA_CFLAGS=-m64 or EXTRA_CFLAGS=-m32 as appropriate for cross-builds.
 #
+# Define EXTRA_BPF_FLAGS="--sysroot=<path>" or other custom include paths for
+# cross-compiling BPF skeletons
+#
 # Define EXCLUDE_EXTLIBS=-lmylib to exclude libmylib from the auto-generated
 # EXTLIBS.
 #
@@ -1244,7 +1247,7 @@ endif
 $(SKEL_TMP_OUT)/%.bpf.o: $(OUTPUT)PERF-VERSION-FILE util/bpf_skel/perf_version.h | $(SKEL_TMP_OUT)
 $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h
 	$(QUIET_CLANG)$(CLANG) -g -O2 -fno-stack-protector --target=bpf \
-	  $(CLANG_OPTIONS) $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
+	  $(CLANG_OPTIONS) $(EXTRA_BPF_FLAGS) $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
 	  -include $(OUTPUT)PERF-VERSION-FILE -include util/bpf_skel/perf_version.h \
 	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@